Tremblor
Source: Quest. Claims to be from a long lost land from the remnants of a massive volcano, but that story seems a little shaky. Elemental type. Unborn Val'kyr: Curse of Doom > Haunt Ikky: Black Claw > Flock till dead Baa'l: Murder the Innocent for the win.
Tremblor is a elemental battle pet obtained from the World Quest Shifting Ground in The Forbidden Reach during the Dragonflight expansion.
Guide awaiting review
Travel to The Forbidden Reach
Head to The Forbidden Reach and locate the World Quest Shifting Ground at 67.25, 12.38. Before engaging Tremblor, defeat two or three Storm-Touched Slyverns in the small area surrounding it to reduce its quality from Legendary down to Epic or Rare — other players in the area benefit from this weakening as well.
Re-weaken Before Each Attempt
The weakening effect from the Storm-Touched Slyverns is temporary and will expire, causing Tremblor to return to its previous quality. Defeat the Storm-Touched Slyverns again before each attempt to keep Tremblor at a lower difficulty tier.
Use Unborn Val'kyr and Ikky
Open with Unborn Val'kyr using Curse of Doom followed by Haunt, then switch to Ikky and use Black Claw followed by Flock. When Ikky dies to Tremblor's Swallow You Whole, finish with a third pet that has a strong hit against elementals — Bloodstone Tunneler, King Viperid, Coral Lashling, or Droplet of Y'Shaarj all work. Use Swallow You Whole twice to close out the fight. If you have Baa'l, replace the third pet slot with Murder of the Innocent.
Collect Tremblor From Rare Defeat
Defeating Tremblor while it is in its Rare form rewards 
Earth Storm Essence, which teaches the Tremblor pet. Defeating it at Legendary or Epic quality does not award the pet.
Tips
Aquatic pets with Water Jet can deal with Tremblor but may be vulnerable to its charge attack, which is strong against aquatics — bring two if using this approach.
Tremblor has a 90% hit rating on some of its attacks, meaning misses occur more frequently than expected.
